1. Field of the Invention:
This invention relates to a film transportation device in cameras in which the film is transported by driving the takeup spool.
2. Description of the Prior Art:
In general, the film transportation in the camera is carried out by driving the sprocket and it is called sprocket drive. In this sprocket drive type camera, as the sprocket shaft is driven to rotate a certain angle, the film is advanced one frame and at the same time the shutter is charged. Therefore, in such camera, regardless of whether or not a film is loaded, the operation of the wind up lever or the like leads to drive the shutter charging mechanism and the like. For this reason, even when there is no film in the camera, a camera release can be made, thus enabling the operator to check the functions of the camera with very ease as its structure is suited for this.
In the case of the sprocket drive, however, the assurance of smooth movement of the film requires two sprockets upper and lower, connected to each other through a sprocket shaft. The bulk and size of the sprocket as a whole is, therefore, very large. Moreover, as the sprocket arrangement has no degree of freedom, the degree of freedom of the design is very limited. Further, the winding up force is applied by the sprockets directly on the perforations so that even a slight catch of the film often results in the breakage of the perforation and in cutting off the film.
To overcome this drawback, the prior art provides a spool drive type camera in which the film is transported by driving the shaft of the takeup spool. Since the winding up force is applied by the spool across the entire width of the film strip, the perforations are protected from damages and the possibility of cutting off the film can be very lessened.
But, in the spool drive type camera, as the film is taken up on the spool, the diameter of the convolution of the film is increased, thus gradually differentiating the length of film taken up on the spool by one revolution of the spool. Because of this, the maintenance of the constant length of the film taken up requires that the spool be stopped at a different position for every time. Since this is very difficult to achieve, use is made of a sprocket gear arranged to engage one of the upper and lower perforations and upon rotation through a certain angle to control the one-frame advancement of the film, and the operation of the film footage, and of the shutter charging mechanism. With such spool drive type camera, therefore, when the film is not yet loaded, the shutter cannot be charged by operating the wind up lever, and the operator is disabled from checking whether or not the camera well functions until the film is loaded.
Since the sprocket used in the spool drive camera is not necessarily to engage both of the upper and lower perforations, but suffices to engage one of them, it may be positioned at a desired location, for example, in the vicinity of one of the longitudinal margins of the exposure aperture thus contributing to the compactness of the camera. Another advantage is that when a motor drive unit is associated therewith, as the driving torque acts on the entire area of the film, there is no possibility of occurrence of breaking the perforations. Accordingly, the recently developed automatized cameras tend to employ this. But since, as has been mentioned above, the checking of the camera without a film loaded is inconvenient, a measure making it possible must be taken into account. The conventional measure is to provide for a changeover mechanism of complicated structure operating in such a manner that when the film is present, the shutter is charged by the sprocket moving as the film is transported, and when the film is absent, the changeover mechanism is switched to transmit the driving torque to the sprocket by which the shutter is charged.
However, the use of such complicated changeover mechanism increases the production cost, and at the same time necessitates a device for actuating the mechanism in response to detection of whether or not the film is present. The means for detecting the presence of the film is rather easy when in the form of detecting the presence of the film container. But, the shape of the film container differs to some extent, depending upon the kind of film or the like. Therefore, this must be taken into account when designing the detecting device.
An object of the present invention is, therefore, to eliminate the above-described drawbacks and to provide a control mechanism for shutter charge operating in such a manner that as a sprocket rotates along with the film transportation, when the rotation of the sprocket is detected, the rotative force of the sprocket is used to carry out the shutter charge and when the standstill of the sprocket is detected, the winding up driving force is used to carry out the shutter charge.
Another object of the present invention is to perform a one-frame advancement by the one of a gear cooperative with the sprocket and a gear cooperative with the winding up driving force which is imparted with rotative force.